1. Field of the Invention
The present invention relates to a rotary vane type evaporator.
2. Description of the Prior Art
The rotary vane type evaporator has a cylindrical vessel having a heated inner peripheral surface and a rotor disposed in the cylindrical vessel and carrying at least one vane. In operation, the material to be treated in a liquid or viscous state, i.e., the substance to be evaporated and dried, is urged against and spread over the heated inner peripheral surface of the cylindrical vessel by means of the vane or vanes of the rotor, so that a heat exchange is made across the wall of the cylindrical vessel to promote the evaporation of the substance thereby to dry the same.
This kind of evaporator can advantageously be used for drying and solidifying a wide variety of substances, a typical example of which is the effluent from a boiling water reactor which contains sodium sulphate as its major component.
Hitherto, there has been proposed a rotary vane type evaporator in which, in order to urge a substance strongly against the inner peripheral surface of the vessel to improve the efficiency of the heat exchange between the substance and the heated wall of the cylindrical vessel and to enhance the scraping or sweeping action performed by the outer ends of vanes, each vane is pivotally connected at its inner end to a point on the rotor offset from the axis of rotation of the rotor so that the vane end is strongly urged against the inner surface of the cylindrical vessel by the centrifugal force which is generated as the rotor is rotated at a high speed. This type of evaporator is shown, for example, in Japanese Patent Publication No. 38-6074 (6074/1963), particularly in FIGS. 9-11. More specifically, in the embodiment shown in FIGS. 10 and 11 of this Publication, each vane is provided with a weight for increasing the centrifugal force acting on the vane. The Japanese patent publication referred to above corresponds to U.S. Pat. No. 3,130,108 by Albert W. Eckstrom and James G. Moor.
The use of the weight, however, raises a new problem, although it is effective in enhancing the pressure at which the substance to be treated is urged against the inner peripheral surface of the cylindrical vessel. Namely, the rate of wear of the outer end of the vane is increased due to the increase of the pressure at which the outer end is urged against the inner peripheral surface of the cylindrical vessel, resulting in a more frequent renewal of the vane. The frequency of the interruption of operation of the evaporator is increased to disadvantageously lower the rate of operation of the evaporator.